The circulation of blood in which blood passes through the heart twice during one complete circuit is called as double circulation.
The heart of man is four-chambered. The left side of the heart is made up of left atrium or auricle (receiving chamber anterior region) and the left ventricle (pumping chamber posterior region). These two chambers are guarded by a bicuspid valve, opening towards the ventricle. The right half of the heart is made up of right atrium or auricle (on the anterior region) and right ventricle (on posterior region) these two chambers are guarded by the tricuspid valves opening towards ventricle. The valves ensure unidirectional flow of blood and prevent its backflow. The right half of the heart receives deoxygenated blood from body and the left half receives oxygenated blood from the lungs. The two halves are separated by the atrioventricular septum ensuring absolutely no mixing up of the blood between these two halves.
The oxygenated blood leaves the left ventricle through the aorta (the great vessel) and reaches various parts of the body through its branches, arteries and capillaries. After this blood is deoxygenated in cells and takes up CO2, it is returned back into the right side of the heart by venules and veins forming larger veins, the superior and inferior vena cava into the right auricle.
From here it enters the right ventricle and is pumped into the lungs through the pulmonary artery. (Aorta & pulmonary artery are guarded by semilunar valves). Later the oxygenated blood from lungs is, carried back into the left auricle or atrium by pulmonary- veins. From the left auricle it enters the left ventricle and from here blood is pumped into the aorta for distribution.
Thus blood circulates (without mixing up) continuously in the human heart and passes it twice during one complete circuit keeping the impure or deoxygenated blood separate from the oxygenated or pure blood during the circulation. This is known as double circulation.
